Cameras and Lenses

blackout

I have seen this term popping up recently. I am wondering whether we are all describing the same thing.

For me, I noticed some binoculars I tried in store have extra long eye relief while the eyecup extension is not matching to that ER. As a result, I see blackout if I push my eyes against the eyecups. It goes away if I move slightly away from eyecups. Not sure if you have similar experience? Let's share and hear it.

[QUOTE=NWBirder;1314281]Not sure if you have similar experience? Let's share and hear it.[/QUOTE]

That is precisely what the blackouts are about. It occurs when the eye pupil is closer to the eyepiece than the correct eye-relief distance, and it is practically a shadow that the eye iris casts on the retina.

The risk for blackouts is increased if:
- the eyepiece has a long ER *and* a wide apparent field of view (often these are mutually exclusive)
- the lighting conditions are good and the eye pupil is small (sunglasses reduce the risk, but make you look silly )
- the eyecups don't extend far enough and/or the eye sockets are not very deep
- there is a phenomenon called spherical aberration of exit pupil ("SAEP"), which I have not seen very well explained, but which probably means that the ER is not "flat" in some eyepieces (like the "notorious" Nikon SEs). This may make it difficult to find the correct eye placement.

Best regards,

Ilkka

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